LONDON, Jan 17 (Reuters) - A big wave of H7N9 bird flu cases and deaths in China since the start of 2014 is a reminder that emerging flu strains need constant surveillance if the world is not to be caught off guard by a deadly pandemic.

At least 24 H7N9 flu infections and three deaths have been confirmed in the past week by the World Health Organisation (WHO), a dramatic increase on the two cases and one death reported for the four-month summer period of June to September.

"There's now a clear second wave of this virus," said Jake Dunning, a researcher at Imperial College London who has been monitoring the outbreak.

While the winter flu season means an increase in infections is not unexpected, it also raises the risk of the virus mutating and perhaps getting a chance to acquire genetic changes that may allow it to spread easily from one person to another.

The H7N9 bird flu virus first emerged in March last year and has so far infected at least 170 people in China, Taiwan and Hong Kong, killing around 50 of them.

Many but not all of the people infected have had previous contact with poultry or other birds, so for now, the fact that this virus has apparently not adapted to easy human-to-human transmission is one of the main features keeping a pandemic emergency response on hold.

Yet the strain already has several worrisome features, including a limited capability to spread from one person to another.

CLUSTERS

Several clusters of cases in people who had close contact with an initially infected person have been reported in China. A scientific analysis of probable H7N9 transmission from person to person, published last August, gave the best proof yet that it can sporadically jump between people.

A separate team of researchers in the United States said in December that while it is not impossible that H7N9 could become easily transmissible from person to person, it would need to undergo multiple mutations to do that.

Another alarm was sounded, also last month, when scientists said they had found that a mutation in the virus can render it resistant to a key first-line treatment drug without limiting its ability to spread in mammals.

WHO chief spokesman Gregory Hartl told Reuters the United Nations health agency had noted the rapid increase in infections in the past few weeks and is keeping a watchful eye.

"So far we haven't seen anything that cause us to change our risk assessment," he said from WHO's Geneva headquarters

The WHO's current stance, based on its December 20 assessment, is that five small family clusters have been reported but "evidence does not currently support sustained human-to-human transmission of this virus."

"The current likelihood of community-level spread... is considered to be low," it says.

Flu viruses, however, often put on their biggest show of strength in the cold winter months of January and February.

And with more of the virus circulating in wild birds, poultry and in the larger numbers of people infected in China and elsewhere, the new strain now has more opportunity to adapt and mix with other strains that may give it pandemic potential.

MIX AND MINGLE

Peter Openshaw, director of the Centre for Respiratory Infection at Imperial College London, said the rising toll of infections and deaths is "a signal for concern" because "historically what has happened in major outbreaks is there are occasional, sporadic cases and then it starts to build".

"But whether it means that there is any change in the virus' behaviour is another important question. If it were changing the way it is behaving, that would be more alarming," he told Reuters.

Early gene analysis work on the emerging H7N9 virus in April last year found it had already been circulating widely but went undetected. During that activity, it had also acquired significant genetic diversity, making it more of a threat.

Scientists warned then that its genetic diversity showed the H7N9 virus has an ability to mutate repeatedly and was likely to continue doing so.

Dunning noted also that H7N9 is now more likely to meet and potentially mix with other seasonal flu virus strains such as H1N1 and H3N2, which are circulating widely among people in China at the moment.

"When you get hybrid viruses forming, that tends to occur in other species, but there is always the potential for it to happen in humans," he said. "So that is a theoretical concern."

Hartl agreed that the opportunity for the virus to take on more features and capabilities is now greater, but stressed that such changes may not necessarily present fresh dangers.

"Mutations happen all the time," he said. "And while yes, the more virus there is, the more mutations could happen, it's also true that almost all of these mutations are benign." (Reporting by Kate Kelland; Editing by Tom Heneghan)